You might see this error:
Create failed for User... The login already has an account under a different user name. 15063
Check the users mapped to the target database. You might be seeing this error because the login is already mapped to the database.
Run this query in your database:
select l.loginname as [login name],u.name as [user name] from DatabaseName..sysusers u inner join master..syslogins l
And you will see a list of user mappings.
DOMAIN\user1 - DOMAIN\user1
DOMAIN\user2 - DOMAIN\user2
DOMAIN\user3witherror - dbo
Double check the dbowner of the database.
The owner is probably mapped to the user3witherror.
sp_changedbowner 'newowner'
re-map your login DOMAIN\user3witherror to the database
(double click on the login -> User Mapping -> check the database -> check the permission level -> OK
